A patch of moss gone brown and crisp looks finished, and people dig it out or throw it away. Most of the time that is a mistake, because moss does not die when it dries; it shuts down and waits, and a good soaking brings it back.
Why brown moss is usually just dormant
Moss can let its water content fall to almost nothing, going dormant in a desiccated state and resuming life when moisture returns. In that state it looks dead, brown and brittle, but the cells are intact and merely paused. This is how it survives drought on a wall or a roof, and it is why a summer-scorched patch greens up again after autumn rain. The default assumption with brown moss should be that it is resting, not gone.
How a plant survives drying out
Most plants die if they lose much of their water, because their cells collapse and cannot recover. Moss belongs to a small group that does the opposite. It is poikilohydric, meaning its water content tracks the air around it, and it can dry until it is barely moist at all and still live. As it dries it packs its cells down in an orderly way and switches off, and on rewetting it repairs the ordinary wear of drying and starts working again within minutes to hours. Having no roots, it neither stores water nor pulls it from the ground; it takes moisture straight in over its whole surface whenever that surface is wet. That is the whole reason a wall or roof moss can ride out weeks of drought that would kill a bedding plant, and why brown does not mean dead.
Bringing it back
Rehydration is simple. Give the moss a thorough wetting with rainwater or low-mineral water and keep it damp, by misting daily or, for a detached piece, by soaking it in a bowl for a quarter of an hour. Within a day or two living moss starts to swell, soften and green from the tips. Set it somewhere shaded and humid while it recovers rather than back in the sun that dried it, and be patient over a week or so; a badly desiccated cushion can be slow to wake fully.
When it really is dead
Moss does have limits. Prolonged drought, baking heat, being smothered under wet leaves until it rots, or a dose of mosskiller will genuinely kill it, and dead moss stays brown, goes slimy or simply crumbles away without ever greening after repeated wetting. If a fortnight of consistent moisture brings no flush of green anywhere in the patch, accept that it has gone and start fresh, as in the growing guide.
Stopping it drying so hard next time
Reviving moss is easy enough that the better aim is to spare it the ordeal. Repeated hard drying is not free; each severe cycle costs the plant a little, and a patch that dries to a crisp every summer grows more slowly than one kept steadily damp. Where you can, move a treasured cushion into more shade, shelter it from drying wind, and water it with rainwater through the driest spells, little and often rather than an occasional flood. In a container or terrarium the same logic says keep the humidity up and the light soft. Moss forgives a great deal of neglect, but it rewards a damp, shaded home by staying green when the untended patches have shut down for the season.
Reviving moss bought dried
Dried sphagnum and some dried sheet mosses sold for crafts and orchids can sometimes be coaxed back to life, since fragments may still be viable, but it is hit and miss; preserved moss, treated with glycerine, is dead for good and will never grow. If you want living moss, start with living moss or fresh fragments rather than counting on reviving a bag of the dried stuff.